1. Field of the Invention
The present invention relates to an inorganic fiber sinter having high strength and toughness and to a process for producing the sinter. More particularly it pertains to an inorganic fiber sinter to be used mainly for internal-combustion engine parts such as a piston ring and an auxiliary combustion chamber, rocket engine parts such as a nose cone and a nozzle, and the like, and a process for producing the sinter.
2. Description of the Prior Art
There have been developed inorganic fiber-reinforced ceramics and inorganic fiber-reinforced glasses as a material for high-temperature service having high toughness and high heat resistance. Among them, silicon carbide fiber-reinforced lithium silicate glass is characterized by its having a fracture toughness value K.sub.IC as high as 10 to 27 MNm.sup.-3/2 and its capability of maintaining its strength up to 1100.degree. C. in the air. However, this glass involves the problem that the strength thereof markedly decreases at 1200.degree. C. or higher because of the insufficient heat resistance of the matrix thereof.
Accordingly, if the content of the matrix can be extremely reduced, that is, if the volumetric content of the fiber in the fiber-reinforced composite material can be considerably increased, it is expected that the aforementioned problem can be solved.
However, according to the conventional processes known as the means for producing long fiber-reinforced ceramics or long fiber-reinforced glasses, such as the CVI process, sol-gel process or slurry impregnation process, the volumetric content of the fiber is generally limited to 70% as the upper limit. Thus it is recognized that an attempt to increase the volumetric content of the fiber so as to exceed the above limit tends to bring about nonuniform distribution of the reinforcing fibers in the composite material, thus impairing the uniformity of the composite material.
In the Japanese Patent Publication No. 39468/1990 filed by the present inventors, there is disclosed a composite material of fiber-reinforced ceramics which comprises an inorganic fiber and a reinforcement, said inorganic fiber comprising:
(1) an amorphous substance consisting substantially of Si, M, C and O,
(2) an agglomerate consisting substantially of crystalline ultrafine particles of .beta.-SiC, MC, solid solution of .beta.-SiC with MC and/or MC.sub.1-x, and amorphous SiO.sub.2 and MO.sub.2, or
(3) a mixture of the above amorphous substance (1) and agglomerate (2),
wherein M is Ti or Zr and x is 0 to less than 1. The composite material contains a carbide, nitride, oxide, glass ceramic or the like as the matrix.
In the above-mentioned patent, there are disclosed alumina, silica, magnesia, mullite and cordierite as specific examples of the oxide ceramics, and borosilicate glass, high silica glass and aluminosilicate as specific examples of glass ceramics. The sinters described in the aforesaid patent are obtained by heating a mixture of an inorganic fiber and a powder for matrix having a composition different from the fiber and therefore, limited in regard to the strength and toughness.
In Japanese Patent Laid-Open No. 74571/1990 is disclosed a sinter obtained by heating a laminate consisting of the above inorganic fiber alone at a high temperature and having surpassingly high flexural strength and toughness as compared with the conventional inorganic fiber-reinforced ceramics.
In the process for producing the above-mentioned sinter, lumpy, flaky or acicular crystals are formed by heat-sintering a laminate at a high temperature and the gaps between the inorganic fibers are filled in by deforming the cross section into a polygon. In order to obtain a structurally uniform sinter, it is necessary in the sintering step to uniformly remove from the surface and inside of the laminate the gases produced by the decomposition of the inorganic fibers constituting the laminate. However, as the laminate gets bulky, it becomes difficult to remove the gases uniformly from all the parts of the laminate, resulting in a nonuniform sinter different in the structure between the inside and surface thereof. Consequently the resultant sinter is apt to be lowered in strength and toughness.